The invention relates to rain gauges of the type comprising a stationary reception funnel, having at its lower part a water discharge nozzle at the input of which a filter is disposed.
The rain gauges used practically universally by meteorological services are of the above defined type. The nozzle of the fixed reception funnel is placed above a receptacle arranged to rock on a support plate and having two symmetrical compartments alternatively receiving water flowing through the nozzle. The receptacle rocks about the horizontal edge on the plate when the amount of water contained in a compartment reaches a given value. This compartment then discharges its contents outside and the other compartment begins to receive any water which flows from the funnel. A switch, generally formed by a mercury bulb with contacts which is fixed to the receptacle indicates, by its open or closed state, the position of the receptacle. From the knowledge of the cross-sectional area of the input of the funnel and from the amount of water which causes the receptacle to rock, the amount of precipitation can be determined by counting the number of times the switch has operated and, by detecting the times of rocking with respect to data delivered by a clock, the distribution thereof in time.
Many rain gauges are located in isolated stations and are only inspected at long intervals. Between two inspections, the filter, which is intended to retain the impurities while letting the rain water or the water from molten snow pass, may be progressively clogged by debris such as leaves, dust and insects. Such progressive clogging is not detected until the first inspection which follows it. Consequently, it is impossible to know how long the filter has been clogged and to determine how long the recorded has been doubtful.